Without limiting the scope of the invention, its background is described in connection with the packaging of integrated circuits, as an example.
Bumped silicon chips, also known as flip-chips, use a solder bump or ball to solder the electrical connections that interconnect integrated circuits. Flip-chip arrangements avoid the problems of breakage and lack of planarity encountered with conventional integrated circuit packages that are wire-bonded. Solder bumps or balls allow direct coupling between the pads on the silicon chip and matching contacts on the substrate. The flip-chip is aligned to the substrate and all connections are made simultaneously by reflowing the solder.
The use of solder bumps on the underside of silicon chips has led to the need to underfill the gap between the silicon chip and the substrate following reflow of the solder bumps. The underfill increases the mechanical integrity and reliability of integrated circuit packages, but requires a time-consuming series of passes in which underfill polymer in liquid form is added to the periphery of the silicon chip. With each addition of underfill polymer, the underfill is built-up over the solder bumps in the gap by capillary action. The entire sequence can take up to 3 minutes per integrated circuit package.
Typically, an underfill polymer is dispensed onto each of the four adjacent sides of bonded flip-chip integrated circuits (ICs). The liquid underfill polymer is drawn into the gap by capillary action. The duration of each flow cycle can last as long as twenty minutes depending on the number of passes and the size of the gap. Furthermore, the sides on which the underfill polymer out-flow are substantially less homogeneous than the in-flow side. A non-symmetric underfill geometry causes stress gradients leading to premature failures of the integrated circuit package.
The inability to provide a uniform underfill has frustrated attempts to provide flip-chip mounting techniques that are reliable in a mass production environment. The types of flip-chip automation that would benefit from a reliable means of dispersing underfill polymer include: in-line automation, repeatable automatic process control, high throughput productions of flip-chips on substrates such as printed circuit boards (PCB), flip-chip multi-chip modules, or carriers to form flip-chip chip carriers, and high volume production of flip-chip chip carriers on PC boards and ceramic substrates.
Yet another solution to the problem of improving the speed of automation of underfilling the gap between the silicon chip and the substrate in flip-chip configuration has been the use of alternating cycles of vacuum and pressure. U.S. Pat. No. 5,203,076 issued to Banjeri, et al. discloses such a method. The Banjeri patent, however, fails to address the need to increase the efficiency in the automation process, as liquid underfill polymer must still be added in repeated cycles.
U.S. Pat. No. 5,073,816 (INMOS Limited) discloses an apparatus and method for packaging semiconductor devices aimed at reducing package size and providing improved heat dissipation over the packages in the prior art. While the specification describes an apparatus and method for flip-chip packaging, it fails to address the need to automate the underfill process.
A need has arisen for a simple, effective apparatus and method for providing a uniform liquid underfill polymer in a manner that avoids the difficulties of prior infiltration methods. A need has also arisen for increasing the speed and automation of the underfill process.